1. Field of the Invention
The present invention relates to a vibratory mechanism and a vibratory roller.
2. Description of the Relevant Art
A vibratory roller is mainly used for a compaction of an embankment in a construction site, such as a highway or a dam, or an asphalt pavement of a road.
The compaction using the vibratory roller is performed while vibrating a vibratory roll (roll). Thus, the ground to be compacted is densified in a very dense state. As an example of a vibratory mechanism that is provided within the vibratory roll and causes a vibration of the vibratory roll, the mechanism that causes vibration by rotating a vibratory shaft provided with an eccentric weight has been known.
Here, as an example of a vibration state of vibratory roll, two types of vibration state have been known. One is “standard vibration” which is a vibration that the vibratory roll vibrates in all radial directions thereof. The other is “horizontal vibration”, which is the vibration that the vibratory roll vibrates in the direction tangential to the circumference of the vibratory roll.
In the mechanism disclosed in U.S. Pat. No. 4,647,247, is a switching unit, by which the vibration state of the vibratory roll is changed to/from the standard vibration from/to the horizontal vibration.
In FIGS. 10A and 10B of U.S. Pat. No. 4,647,247, a total of two vibratory shafts are provided within the vibratory roll. One of the vibratory shafts is provided at an opposite position across the center of the vibratory roll with respect to the other vibratory shaft. Each of the vibratory shafts is provided with an eccentric weight, and the eccentric weight of at least one of the vibratory shafts is rotatably attached to the vibratory shaft.
In this U.S. patent, if the relative phase angle between eccentric weights in case of rotation in one direction of the vibratory shaft is denoted by 0°, the relative phase angle between the eccentric weights in case of rotation in the other direction of the vibratory shaft is 180°.
When vibrating the vibratory roll under standard vibration or horizontal vibration, the vibratory roll should be vibrated at the suitable amplitude for respective vibration states.
FIG. 4 is an explanatory view showing the vibration of a vibratory roll equipped with a pair of vibratory shafts in case of standard vibration.
In this vibratory roll, an eccentric weight of the same shape is provided to respective vibratory shafts, which are rotated in accordance with a rotational torque supplied from a power supply mechanism (not shown). Thus, respective eccentric weights are rotated in the same direction at the same angular position.
In this occasion, the vibratory force directed away from the center of the vibratory roll is caused, and the direction thereof changes sequentially according to the angular position of eccentric weights. Here, if it is focused on the element vertical to a ground from among all elements of the vibratory force, and the vibratory force thereof is denoted by F, the vibratory force F is indicated by a following formula.F=2·m·r·ω2·sinωt where
m is a mass of an eccentric weight
r is a distance between the center of the vibratory shaft and the center of gravity of the eccentric weight
ω is an angular velocity of vibratory shaft.
Here, m·r is defined as eccentric moment (hereinafter m·r is indicated as “mr”).
Thus, a ground can be indicated as a model of spring, which has a predetermined spring constant K and which acts in a perpendicular direction with respect to the contact surface between the vibratory roll and a ground.
When vibratory force F is periodically working on the vibratory roll whose mass is M0, if spring constant K is regarded as a negligibly small value by assuming that a ground is quite loose, the equation of motion is shown by a following formula.2·mr·ω2·sin ωt=M0·d2y/dt2 where
y is a displacement in ups-and-downs directions.
Then, the following formula is obtained from this formula.y=(−2·mr/M0)·sin ωt 
Thus, the amplitude a1 in the ups-and-downs directions of the vibratory roll in case of standard vibration can be shown by a following formula (1).a1=2·mr(standard vibration)/M0  (1)
In this formula, “mr (standard vibration)” means that the eccentric moment in case of standard vibration.
FIG. 5 is an explanatory view showing the vibration of a vibratory roll equipped with a pair of vibratory shafts in case of horizontal vibration.
A vibration proof rubber provided between the vibratory roll and a frame (not shown) of the vibratory roller can be indicated as a model of a spring, which has a predetermined spring constant K1 and which acts in a horizontal direction with respect to a shaft center O′ of the vibratory roll.
A ground can be indicated as a model of a spring, which has a predetermined spring constant K2 and which acts in a horizontal direction with respect to the contact surface between the vibratory roll and a ground.
When a periodic torque T is acting on a moment of inertia I around the shaft center O′ of the vibratory roll, which is supported by the spring of spring constant K1 and the spring of spring constant K2, the equation of motion of this case is as follows.p·2·mr·ω2·sin ωt=I·d2θ/dt2 where
p is a distance between the shaft center O′ of the vibratory roll and the center of the vibratory shaft.
Here, respective spring constant K1 and K2 are regarded as a negligibly small values by assuming respective springs are quite loose.
If the radius of the vibratory roll is denoted by R, a displacement y in a horizontal direction with respect to the contact surface between the vibratory roll and a ground can be indicated as y=R·θ, on regarding θ as a slight angular displacement. Thus, a following formula can be obtained.p·2·mr·ω2·sin ωt=(I/R)·d2y/d t2 
Then, by performing a formula translation based on y, a following formula is obtained from this formula.y=−((R·p·2·mr)/I)·sin ωt 
Thus, the amplitude a2 in a horizontal direction with respect to the contact surface between the vibratory roll and a ground in case of horizontal vibration can be shown by a following formula.a2=R·2·p·mr(horizontal vibration)/I  (2)
In this formula (2), “mr (horizontal vibration)” means that the eccentric moment in case of horizontal vibration.
Here, a mass M0 of a vibratory roll, a radius R of the vibratory roll, and a moment of inertia I around the shaft center O′ of the vibratory roll are determined depending on a dimension of the vibratory roll. Therefore, it is required that the eccentric moment mr (standard vibration) can be determined freely for controlling the amplitude a1 in case of standard vibration to the desired value.
Additionally, it is required that at least one of the distance p and the eccentric moment mr (horizontal vibration) can be determined freely for controlling the amplitude a2 in case of horizontal vibration to the desired value. Here, the distance p is a distance between the shaft center O′ of the vibratory roll and the center of the vibratory shaft.
In the vibratory roll, however, since the vibratory shaft is provided within the vibratory roll, there is a limitation of the distance p (see FIG. 5). Thus, the eccentric moment mr (horizontal vibration) has a great influence on the amplitude a2 in case of horizontal vibration.
Therefore, it is preferable that the eccentric moment in case of standard vibration is different from the eccentric moment in case of horizontal vibration, for establishing the amplitude a1 of standard vibration and the amplitude a2 of horizontal vibration at respective suitable values.
In U.S. Pat. No. 4,647,247, as described above, a total of two vibratory shafts, each of which is provided with an eccentric weight, are stored within the vibratory roll, and the eccentric weight of one of the vibratory shafts is rotatably attached to the vibratory shaft. Therefore, the angular position between eccentric weights varies depending on the rotation direction of the vibratory shaft, but the eccentric moment in case of standard vibration is the same as the eccentric moment in case of horizontal vibration. Therefore, it has been difficult to control the amplitude of the eccentric moment to respective suitable amplitudes for the standard vibration and the horizontal vibration.
Therefore, the vibratory roller that can control the amplitude of the vibratory roll to the desired value for the standard vibration or the desired value of the horizontal vibration has been required.